Taiwan is a country of about 24 million people, 81 miles off the coast of mainland China. As of late April 2020, Taiwan had about 330 confirmed cases of coronavirus disease 2019 (COVID-19) and 6 deaths. By comparison, the US had about 1 million confirmed cases of COVID-19, and 60 000 deaths.

In this issue of JAMA Internal Medicine, there is a remarkable report from Taiwan on the use of contact tracing and virologic polymerase chain reaction testing to assess the transmission dynamics of COVID-19 in the country’s initial 100 confirmed cases.1 Among 2761 close contacts of the 100 cases, confirmed between January 15 and March 18, 2020, Cheng et al report that there were 22 paired-index secondary cases and an overall secondary clinical attack rate of 0.7% (95% CI, 0.4%-1.0%).1

The study has important messages for the control of COVID-19 throughout the world. First, people with COVID-19 were found to be most infectious to others before and within 5 days of symptom onset. Within 5 days of symptom onset, the attack rate was 1.0% (95% CI, 0.6%-1.5%). With exclusive presymptomatic exposures, the attack rate was 0.7% (95% CI, 0.2%-2.4%), and with exposures 6 days or more after symptom onset, there were 0 cases from 852 contacts (95% CI, 0%-0.4%).1

These findings underscore the pressing public health need for accurate and comprehensive contact tracing and testing. Testing only those people who are symptomatic will miss many infections and render contact tracing less effective. The finding that asymptomatic people and those with minimal or fewer symptoms early in infection are those most likely to transmit COVID-19 strongly argues for maintaining social distancing and having people wear face masks to reduce the potential for transmission. Solely isolating patients symptomatic with COVID-19 will fail to control transmission during the infected but asymptomatic stage.

Second, the study underscores the many things that Taiwan has done right in proactively and rapidly responding to COVID-19.2 It is impressive, even astounding, that Taiwan not only conducted robust contact tracing and testing on the first 100 confirmed cases, but also quickly and comprehensively reported the results, thus meaningfully advancing knowledge of the transmission dynamics of the virus.1 Unfortunately, widespread testing was not available in the US in February 2020, hampering the ability to identify people who were COVID-19 positive.

A first step for the US and other nations in “reopening” society is to have sufficient testing and contact tracing such that the outbreaks that will inevitably occur as social restrictions are removed can be successfully contained. Beyond this, even when “reopening,” social distancing throughout society and the wearing of face masks should be maintained to the maximum extent possible until there is a vaccine or effective treatment.

In Taiwan authorities' own words: "Initiation, Integration, and Innovation: The 3 "I"s enable the NHCC to respond to different kinds of crisis such as communicable disease outbreaks, biological disasters, bioterrorism attacks and medical emergencies.

Under central command system, we can coordinate with officials at all levels and can also provide comprehensive information on public health emergency for decision-makers to take timely actions. Therefore, we can respond to the crisis more effectively and safeguard the health of our citizens."

I found this study to be more complete than the other contact tracing articles being cited. I have a question about the conclusions, which do not seem to support the idea that asymptomatic and pre-symptomatic transmission play a major role in the overall infectivity. From my understanding, 13 cases stemmed from 6 of the subjects who had the most severe disease-this is based on the fact that the authors refer to 786 contacts of the 6 severe cases (defined as severe pneumonia and sepsis) as having risk ratios of 3.76 and 3.99%, and in table 2, that corresponds to 7 and 6 secondary cases, respectively. This means that the majority of the subjects did not transmit the disease at all. Also, the majority of contacts (1836) were outside household, family or healthcare workers, and yet only 1 of those contacts became infected, and that contact was exposed during the middle of the symptomatic period. None of the 9 asymptomatic subjects transmitted and only 2 out of the 299 contacts who were exposed pre-symptomatically became infected, both of whom were either household, family or healthcare workers according to the breakdown. The missing piece of information is how long the presymptomatic exposures were exposed. The study lists a minimum of 15 minutes of face to face contact with no PPE as the criterion, but from the timeline, it seems that most contacts were exposed for multiple days. From a policy standpoint, it seems that the crucial finding is the lack of asymptomatic and pre-symptomatic transmission outside of close contacts and healthcare workers, and if more studies such as this were conducted, they would inform the debate over which activities are likely to be the main sources of infection. I am confused as to how the major conclusion is that people with fewer symptoms are the most likely to transmit. The study seems to suggest a lower transmissibility among those groups than has been previously suggested.